In JP11-22609A, technology (hereunder, referred to as “prior art”) is disclosed that relates to control for switching a combustion system in an internal combustion engine in which the combustion system of the internal combustion engine can be switched from stratified combustion to homogeneous combustion, or from homogeneous combustion to stratified combustion. Since an air-fuel ratio during stratified combustion is leaner than an air-fuel ratio during homogeneous combustion, switching of the air-fuel ratio accompanies switching of the combustion system. Known methods for switching an air-fuel ratio include a method that gradually changes the air-fuel ratio so that a torque level difference does not arise. However, according to the aforementioned known method, although a torque level difference is lessened, there is the problem that the desired torque cannot be obtained, and because an air-fuel ratio that is not originally intended is used, there is a deterioration in the emissions. The aforementioned prior art has been proposed as a solution to these problems.
According to the aforementioned prior art, at a time of switching from stratified combustion to homogeneous combustion, the target equivalence ratio is switched in a step manner together with the target air amount (target cylinder intake air amount). More specifically, at the same time as the target equivalence ratio is increased in a step manner, the target air amount is decreased in a step manner so that the torque is constant. However, because the actual air amount lags relative to the target air amount, the fuel amount that is decided based on the target equivalence ratio after switching of the combustion system is in excess of an amount necessary to maintain the torque at a constant amount. In the aforementioned prior art, the excess of the fuel amount is balanced with the retardation of the ignition timing to avoid an increase in torque while switching the equivalence ratio with favorable responsiveness in response to switching of the combustion system.
Further, according to the aforementioned prior art, at a time of switching from homogeneous combustion to stratified combustion, only the target air amount is switched in a step manner before switching the target equivalence ratio in a step manner More specifically, only the target air amount is increased in a step manner to increase the air amount in advance, and a target equivalence ratio is decreased in a step manner at a timing at which the actual air amount reaches the target air amount. That is, during a period in which the air amount is increasing with a delay relative to the target air amount, the target equivalence ratio is maintained at the ratio that is used prior to switching of the combustion system. However, when the fuel amount is decided based on the target equivalence ratio prior to switching of the combustion system, the fuel amount is in excess of an amount necessary to maintain the torque at a constant amount. Therefore, in the aforementioned prior art, an increase in the torque prior to switching of the combustion system is avoided by balancing the excess of the fuel amount with the retardation of the ignition timing.
In this connection, in the aforementioned prior art, switching of the combustion system is determined based on the operating state of the internal combustion engine. A specific example of a switching determination that is made based on the operating state is described in paragraph [0042] of the aforementioned patent literature. The aforementioned paragraph [0042] includes the description that “In the case of performing direct fuel injection in a spark-ignition type engine such as a gasoline engine to thereby switch between stratified combustion and homogeneous combustion, operating ranges exist in which homogeneous lean-burn combustion cannot be used, such as when idling or in an extremely low load operating state. If a request to switch from stratified combustion to homogeneous combustion is generated as a result of application of a high load such as by an air conditioning system in these operating ranges, it is necessary to switch from ultra-lean stratified combustion at an air-fuel ratio of 30 or more to homogeneous stoichiometric combustion at the theoretical air-fuel ratio”. Further, as described in paragraph [0039] therein, according to the aforementioned prior art the target torque is maintained at a constant amount when switching the combustion system. In addition, as described in paragraph [0041] therein, switching of the combustion system in a state in which the torque is kept constant is achieved by retarding the ignition timing to avoid an increase in the torque due to an excess in the fuel amount. As will be apparent from the contents of the aforementioned official gazette, the above described prior art is technology that relates to switching control for a combustion system in a steady state in which the target torque is made constant.
When air-fuel ratio switching control for switching from operation according to the theoretical air-fuel ratio to operation according to an air-fuel ratio that is leaner than the theoretical air-fuel ratio or for performing switching in the opposite direction is considered, it is found that such switching control may be performed not only in a steady state but also in a transient state. For example, in the case of an internal combustion engine in which operation according to a leaner air-fuel ratio than the theoretical air-fuel ratio is possible, even when operation according to the theoretical air-fuel ratio must be selected in a high load region, if the torque is to be decreased in response to a deceleration request from the driver, the operation mode can be switched to operation according to a leaner air-fuel ratio than the theoretical air-fuel ratio. Conversely, even when operation according to a leaner air-fuel ratio than the theoretical air-fuel ratio is selected in a low-to-medium load region, if the torque is to be increased in response to an acceleration request from the driver, in some cases the necessity arises to switch the operation mode to operation according to the theoretical air-fuel ratio.
As described in relation to the aforementioned prior art, in the case of control for switching the air-fuel ratio in a steady state, it is desirable to switch the air-fuel ratio with favorable responsiveness while maintaining the torque at a constant level. Similarly, with respect to the control for switching the air-fuel ratio in a transient state, it is desirable to switch the air-fuel ratio with favorable responsiveness while smoothly increasing or decreasing the output torque of the internal combustion engine in accordance with an increase or decrease in the requested torque that is requested by the driver through operation of an accelerator pedal. However, it is not easy to apply the aforementioned prior art to control for switching the air-fuel ratio in a transient state. This is because the aforementioned patent literature only describes a technique for switching the air-fuel ratio (equivalence ratio) that is based on the premise that the torque is kept constant, and does not include any description with regard to a technique for switching the air-fuel ratio with favorable responsiveness while smoothly decreasing or increasing the torque.
Note that technology disclosed in JP2008-038865A can be mentioned as an example of technology for suppressing the occurrence of torque shock when switching the operation mode between operation according to a theoretical air-fuel ratio and operation according to an air-fuel ratio that is leaner than the theoretical air-fuel ratio. However, the technology disclosed in the aforementioned JP2008-038865A is also technology that is designed for switching the air-fuel ratio with favorable responsiveness while keeping the torque constant, and there is no description therein with regard to a technique for switching the air-fuel ratio with favorable responsiveness while smoothly decreasing or increasing the torque at a time of transient operation. Although JP2010-223122A discloses technology that relates to torque demand control for determining operation amounts of respective actuators in accordance with the requested torque, there is no description therein with respect to switching of the air-fuel ratio.